The anthracyclines of the daunorubicin group, such as doxorubicin, carminomycin and aclavinomycin, are among the most widely employed agents in antitumoral therapy. They are polyketides produced by various strains of Streptomyces (S. peucetius, S. coeruleorubidus, S. galilaeus, S. griseus, S. griseoruber, S. viridochromogenes and S. bifurcus).
Doxorubicin is only produced by S. peucetius var. caesius. The type strain S. peucetius var. caesius IMRU 3920 (hereinafter abbreviated to "S. peucetius 39.20") is publically available and is described in U.S. Pat. No. 3,590,028. S. peucetius 3920 has been deposited at the Institute of Microbiology of the Rutgers University, US, receiving the index number IMRU 3920. This strain and its mutants obtained by classical mutagenic treatments are resistant to high levels of doxorubicin.
The study of the mechanisms involved in the resistance to these substances is crucial for two main reasons:
a) There are many examples in which the genes involved in the biosynthesis of secondary metabolites are all clustered together with at least one resistance gene: for example oxytetracycline (Rhodes P. M., Hunter I S., Friend E. J. and Warren M., 1984, Trans Biochem Soc 12, 586-587), erythromycin (Stanzak R., Matsushima P., Baltz R. H. and Rao R. N., Biotechnology vol 4, March 1986, 229-232), tylosin (Fayerman J. T., Biotechnology vol 4, September 1986, 786-789) and tetracenomycin (Motamedi H., Hutchinson C. R., Proc Natl Acad Sci U.S.A., vol 84, 4445-4449, 1987). Cloning the biosynthetic genes can be useful with a view to altering pathways to produce different molecules or to overcome bottlenecks present in the biosynthesis routes thus augmenting the productivity of the strain.
b) The resistance itself can be implied in the regulatory mechanisms so that changing the resistance levels (i.e. augmenting the gene dosage) the productivity of the strain can be improved. This is an old idea usually performed via the classical methods of mutagenesis and random screening, but renewed by the utilisation of rDNA methods (Craveri R. and Davies J. E., The Journal of Antibiotics, January 1986, 128-135).